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B7008 - What is the purpose of property lists? [Part 1]

by - Joseph Ganci

I'm quite happy using lists in general but I have never quite figured out property lists.  Can someone please explain what these are, how they differ from ordinary lists and why I might want to use them?

Property lists are like linear lists except that you point to locations in the list not by number, but by a property name. For instance, let's say that you have a list of patients, students, customers, etc., and for each you are keeping track of certain items of information, such as name, street address, phone number, city, state, and zip (postal code for those of you outside the USA). You could set up a linear list, such that:

person[1] is where you store the name
person[2] is where you store the street.

But with property lists, you can use the name, coupled with a pound sign, to indicate the location:


The combination of the pound sign with the property name is called a symbol and takes up very little room in memory, unlike a variable, since a symbol only indicates a location in the list. This is true in Director and I assume the same is true in Authorware.

You can mix linear list and property lists. For instance, you can have a list of property lists:

persons[id number][#name]
persons[id number][#street]

There are several functions that deal with lists and property lists. They're all good. One that is often obscured is the function called Symbol. It will convert any string into symbols. This is not for novices! It's used when you want to set, reset, add to, or look for values in a property and you want don't know ahead of time the name of the property.

For instance, let's say you are in charge of a zoo, and you set up each animal in the zoo to have a location in our zoo property list. The value of each one will be how many of that animal your zoo possesses. You start with only three animals:

zoo[#elephant] := 2
zoo[#tiger] := 4
zoo[#slug] := 123

The above sets up our zoo with 2 elephants, 4 tigers, and 123 slugs.

Now let's say you want the user to add a new animal. You have no idea what the user will type. You set up a text entry interaction, then you use the Symbol and the AddProperty functions. The user's animal will be contained in the Authorare system variable EntryText, so it's a relatively simple matter:

animal := Symbol(EntryText)
how many := Random(1 , 50 , 1)
AddProperty(zoo, animal, how many)

You could combine the three lines above into one:

AddProperty(zoo , Symbol(EntryText) , Random(1 , 50 , 1))

The above creates a new location in the property list, assigns the user's entry as its property name, and sets its value to a random number between 1 and 50.

Hope this helps.

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